This study investigated the effects of overcrowded classrooms on teaching and learning process in Ilemela Municipality, Mwanza -Tanzania. The study focused on four objectives namely; to assess the availability of school’s infrastructure, to identify the number of students per class stream, to establish the challenges teachers face when they teach overcrowded classrooms and to suggest ways to overcome those challenges. The study adopted a mixed research and convergent parallel research design. The target population of the study was 34832 including MSEO, Quality Assurance, School heads, teachers, and students and Sample size was 110 respondents 1 MSEO, 1 Quality Assurance, 8 heads of school 40 teachers and 60 students, sampled through probability and non-probability sampling techniques. The study employed questionnaires and interviews to obtain required data from the field. Quantitative data was analysed using descriptive statistics with the help of SPSS version 20 and was presented in tables, while the qualitative data were analysed thematically. The study revealed that while schools were instructed to accommodates 50 students per class, students exceeded 90 within a single stream. The challenges of overcrowded classrooms were, difficult in application and implementation of competence based curriculum, and poor classroom managements. This concludes that overcrowded classroom affect teaching and learning process, since it affects effective classroom management. The study recommended that students from primary schools should be enrolled in other learning institution like VETA. Furthermore, it is recommended that the government and community should build secondary in existing primary schools to enable students from primary A to joined secondary A on the same school.